1. Technical Field
The present invention relates to a resistance structure, an integrated circuit and a method of fabricating a resistance structure.
2. Related Art
In a semiconductor integrated circuit, circuit components such as resistance elements (resistors), capacitors, transistors and so forth are combined to structure a desired electronic circuit.
Resistance elements include, for example, a resistance element that is constituted of polysilicon and formed on a semiconductor substrate with an insulating layer interposed. In this structure, a potential difference may be produced between the semiconductor substrate and the resistance element, and the resistance value of the resistance element may change. For example, if a voltage V1 is applied to a resistance element r1 and the semiconductor substrate is fixed at a ground potential, the potential difference between the semiconductor substrate and the resistance element r1 varies if the voltage V1 varies, and the resistance value of the resistance element r1 changes. In an electronic circuit in which, for example, an output voltage is regulated by a resistance ratio, such as a voltage division circuit, an amplification circuit, a level-shifting circuit or the like, the resistance ratio may vary in accordance with applied voltages, and errors may be produced in the output voltage.
In Japanese Patent Application Laid-Open (JP-A) No. 2012-109535, a resistance element is disclosed that may suppress resistance value variations caused by potential differences between a semiconductor substrate and the resistance element. Specifically, a resistance structure disclosed in in JP-A No. 2012-109535 is provided with: a resistance element layer that is formed on a semiconductor substrate with an insulating layer interposed; a first electrode that conducts at one end portion of the resistance element layer; a second electrode that conducts at the other end portion of the resistance layer; and a first conductive layer and a second conductive layer that are adjacent to a lower portion of the resistance element layer with an insulating layer interposed and are separated from one another. The first conductive layer is biased by the potential of the first electrode, and the second conductive layer is biased by the potential of the second electrode. According to this structure, variations in the resistance value due to potential differences between the semiconductor substrate and the resistance element layer may be suppressed by the first conductive layer and the second conductive layer.
In the resistance structure recited in JP-A No. 2012-109535, at a gap between the first conductive layer and the second conductive layer, the resistance element is adjacent to the semiconductor substrate. Consequently, the effect of suppressing variations in the resistance value of the resistance element that are caused by potential differences between the semiconductor substrate and the resistance element is weakened. Specifically, in a CMOS semiconductor integrated circuit, the first conductive layer and second conductive layer recited in JP-A No. 2012-109535 are assumed to be realized by well layers. In this case, so that the first conductive layer and the second conductive layer do not short-circuit together, a gap of the order of several μm or tens of μm is provided. At this gap, the resistance element layer may be affected by electric fields caused by potential differences with the semiconductor substrate, and the resistance value may change.
Furthermore, if a width dimension of a resistance element is represented by W and a length dimension is represented by L, it is commonly known that the inconsistency of resistance values between resistance elements is proportional to 1/(L×W)1/2. That is, the smaller the area of a resistance element, the greater the inconsistency of the resistance value, and consequently the lower the accuracy of a resistance ratio. Accordingly, in order to suppress the inconsistency of resistance elements and raise resistance ratio accuracy, it is necessary to increase the length dimension L and the width dimension W of the resistance elements to some extent. However, in a structure that uses conductive layers to prevent resistance value variations due to potential differences between a resistance element and a semiconductor substrate, such as that recited in JP-A No. 2012-109535, the resistance element must be arranged within a region in which the conductive layers are formed. Therefore, it is difficult to significantly increase the length dimension L and the width dimension W of the resistance element. In other words, in a conventional structure, if the length dimension L and width dimension W of a resistance element are to be assured and resistance value inconsistency is to be reduced, measures such as enlarging a region of formation of a conductive layer are required. However, this leads to an increase in the area occupied by a resistance structure that includes this conductive layer.